On Earth, there is no other tornado breeding ground like the US Mid-West. To the south, the Gulf of Mexico provides heat and moisture, to the north, Arctic air masses regularly plunge down over the heating plains of spring and summer, and to the west, mountains provide both dry air and instability. These features combine annually to spawn hundreds of tornadoes over this region.

In the more stable climate of the 20th century, risk posed by these powerful storms was a regular part of Mid-Western life. But now, with humans inducing the world’s climate system to rapidly change, the world’s most violent tornadoes are being put on the extreme weather equivalent of steroids. This amping up the atmosphere to produce more violent storms occurs through three processes: added heat, added moisture, alterations to the jet stream.

Since the 1880s, world temperatures have risen by .8 degrees Celsius. This increase is roughly equivalent to 1/6th the difference between now and the last ice age, but on the side of hot. Moreover, more than half of this increase has occurred over the past three decades. Added heat goes to providing more energy instability in the atmosphere and instability is one of the key ingredients of the supercell thunderstorms that spawn tornadoes.

Another key ingredient to thunderstorm strength is moisture. As the oceans warm, more moisture ends up in the atmosphere. This added moisture held aloft goes to feed many of the thunderstorms that do spawn, making them, on average, larger and more powerful. Larger and more powerful storms, like the 60,000 foot tall and 100 mile wide behemoth that pummeled Oklahoma last Friday, tend to spawn more destructive tornadoes.

The final ingredient adding to the strength of storms over the US Mid-West are changes to the polar jet stream. Over the past decade, massive losses of sea ice in the Arctic have led to a slowing of the polar jet stream. This slowing has caused the jet to, more and more often, stretch out in elongated wave patterns. These high amplitude meridonal waves (Rossby waves) reach higher into the Arctic and dip lower into the tropics. The result is that warm and cold air masses meet more often. This confluence of air masses further adds to the instability already created by higher temperatures.

The image of the jet stream below provides an illustration of this kind of amplified wave in the jet stream. It is exactly the kind of dip that emerged during mid-to-late spring of 2013 and has persisted through early June. While serving as a conveyer belt for the storms plaguing the Mid-West throughout this spring, it was also a source of enhanced atmospheric instability. One that has aided in the spawning of numerous severe tornado outbreaks.

In total, added heat, added moisture, and added instability, all elements caused by human global warming, increase the power of thunderstorms and the intensity of tornadoes. According to Dr. Kevin Trenberth, these changes have resulted in storms being more severe, adding about 5-10% to their intensity. This, in turn has had a compounding effect on thunderstorm and tornado damage.

Trenberth notes:

Warmer and moister conditions are the key for unstable air.

The climate change effect is probably… a 5 to 10 percent effect in terms of the instability and the subsequent rainfall, but it translates into up to a 32% effect in terms of damage.

(It is highly nonlinear).

Trenberth’s observations are born out in Munich Re damage assessments from thunderstorms over the past few decades. In fact, Munich Re tracking shows that Dr. Trenberth’s estimates may well be conservative. What Munich Re has found is that damage from thunderstorms has more than tripled since 1980 :

More intense and severe thunderstorms result in more intense and severe tornadoes. These numbers and assessments by some of the world’s top climate and hazard experts have now been born out in one of the worst episodes of tornado damage in the US. It follows numerous similar episodes that have happened with greater and greater frequency over the past decade. And these events, most certainly, have been made worse by a combination of factors caused by climate change.

What To Expect: Increasing Periods of Intense Storms Broken Primarily by Drought

The heating of the atmosphere, its added moisture content, and the alterations to the northern polar jet stream will result in a number of severe impacts for the US Mid-West. The first is that we can expect the weather to, increasingly, whip-saw between extended periods of hot and dry weather back to periods of increasingly severe weather.

Such whip-sawing will be brought about by the increasing prevalence of high amplitude Rossby-type waves forming over the central US. During periods when the wave pattern is south to north, hot and dry conditions will dominate, leading to more extreme droughts and heat-waves. During periods when the wave pattern is north to south, stormy conditions will dominate, leading to more extreme floods and tornadoes.

This period will likely intensify over the next two to four decades until large pulses of Greenland melt dump massive volumes of cold water into the North Atlantic. At this time, weather will, likely, again shift into an even more extreme pattern dominated by massive and powerful frontal storms scouring the Northern Hemisphere.

These extreme events can be moderated, and possibly prevented, by a rapid reduction and elimination of worldwide greenhouse gas emissions. But under business as usual ‘growth,’ they are almost certain to continue to worsen.

2 Comments

I presumed that these hellacious tornadoes I’ve read about recently were amplified by AGW, and indeed they are, much more so than I realized. As you say – “nonlinear” changes. I did laugh, uneasily, at the little caveat you put at the end of this essay:

“These extreme events can be moderated, and possibly prevented, by a rapid reduction and elimination of worldwide greenhouse gas emissions. But under business as usual ‘growth,’ they are almost certain to continue to worsen.”